Many methods of laser isotope separation have been proposed in the literature recently. The main criteria for successful separation and good yield are: first, initial selectivity which means a spectrum with a well-defined isotope shift; and second, conservation of the initial selectivity in the subsequent stages of separation. The main methods, which have been proposed and carried out to some extent so far are: (1) Selective photoexcitation of atoms, followed by chemical reaction or by photoionization and electrostatic or magnetic separation of the ions. (2) Selective photoexcitation of molecules followed by dissociation (one step predissociation), speeded-up chemical reaction, and photodissociation or photoionization by a second photon. (3) Physical separation by selective beam-pushing by photons. Method (1) has the practical problems of the high cost of making atomic beams. Throughput using method (1) or method (3) is quite low. Molecular methods are more advantageous from the point of view of throughput and the energy needed to form the beam. Photodissociation usually produces reactive fragments which then can destroy selectivity through secondary reactions. Reaction rates usually compete with intermolecular relaxation and photodissociation competes with photoionization in molecules.
Supersonic nozzle beams have been studied over many years; they operate in a collisionless regime. The main practical problem with them is the high pumping rate required. According to Fenn et al, Adv. Chem. Phys. 10, 275 (1966), the origin of the problem is in the nozzle, where, because of the low Reynolds number, there is no adiabatic core left. Therefore, the emerging beam has to be skimmed; further, the beam cannot be passed through a diffuser for high-pressure pumping. Two recent proposals discuss the use of supersonic nozzles in isotope separation without the use of lasers [C. A. Wang, Nature 253, 260 (1975); J. B. Anderson, P. Davidovits, Science 187, 642 (1975)]. Another well-documented method for physical separation of isotopes in supersonic flow is the Becker nozzle process [Becker et al, Angew. Chem. Int. Ed. Engl. 6, 507 (1967)].